Antitumor antibody therapeutics have significantly impacted the treatment of cancer over the past two decades, yet challenges and opportunities remain. For advanced stage solid tumor treatment, complete clinical remissions are infrequent and transient, even with concomitant chemotherapy. A better understanding of the common therapeutic and resistance mechanisms is urgently needed to improve clinical outcomes of this emerging class of monoclonal antibody (mAb) therapeutics. The first clinical successes, Rituximab (anti-CD20), Trastuzumab (anti- HER2) and Cetuximab (anti-EGFR) were initially thought to work exclusively through interruption of their respective downstream signaling pathways. Unexpectedly, however, our own studies demonstrated that Fc-FcR interactions were required for efficacy of these drugs in the mouse, defining an essential role for effector immunity. Consistent with our observations in mice, several clinical studies have since correlated favorable clinical outcomes in cancer patients harboring higher IgG affinity FcR alleles. The FcR-mediated immunological mechanisms have been collectively described as 'antibody- dependent cellular cytotoxicity' (ADCC), a concept that stems from in vitro observations of cytotoxic FcR-bearing innate effectors (macrophages, neutrophils and NK cells) that rapidly kill IgG-coated tumor cells, within hours. Yet in vivo, with the exception of Rituximab-mediated clearance of normal and malignant B cells in the blood (e.g., CLL), there is little evidence for an 'ADCC-like' acute inflammatory response for solid tumor treatment. Instead, the slower kinetics of tumor responses are more consistent with an induced adaptive immune response, elicited as a vaccinal response to combination chemotherapy and antitumor antibodies. In this model, antibody opsonization of apoptotic/necrotic material provides a therapeutic window to modulate the immunological outcome of tumor antigen uptake by antigen presenting cells. Our recent studies demonstrated clinical relevance of this vaccinal pathway in breast cancer patients treated with chemotherapy + trastuzumab. Patients treated with this regimen developed antitumor HER-2 specific immunity, which we found were enriched in patients with favorable clinical outcomes, indicating that passive mAb therapy induces active tumor immunity that may contribute to efficacy. In this proposal, we will further define the importance of the vaccinal pathway mechanistically (Aim 1) and pursue efforts to enhance these effects using two innovative approaches. We will first use antitumor antibodies engineered to selectively engage activating human Fc receptors on dendritic cells (Aim 2), and secondly, we will use pharmacologic strategies to block inhibitory pathways that limit T cell activation (Aim 3). Our Aims are designed to test the hypothesis that removal of negative regulatory pathways could promote this burst of antibody-enhanced antigen delivery, leading to effective anti-tumor cell T cell generation and conversion of a suboptimal antitumor treatment response into meaningful clinical benefit.